I have a project where I have a main board with a possible mezzanine board connected to it through a connector.
The main board has a 12V supply and got a DC/DC for 3.3V voltage (call it 3.3VMAIN) to power components.
The mezzanine board is also provided with 12V supply and got it's own DC/DC for 3.3V voltage (call it 3.3VMEZ).
GND is the same for both boards, connected through connector and attachment screws.
Main board and mezzanine are communicating through some GPIOs and a SPI bus (running 1-5MHz max).
I was wondering:
- If I could connect SPI devices of the main board directly to SPI devices of the mezzanine
- OR if I need to add some open collector buffers due to the fact I don't have the same power supply reference (3.3VMAIN vs 3.3VMEZ)
One of my concern is that I could get some reverse voltage from one the 3.3V rail to the other one in case of the board is not powered on. Second concern is to have open collector buffer with pull-up on all SPI bus lines.